The detrimental effects of breathing cold air, particularly for people with medical problems such as cardiac conditions, angina, and asthma, and the benefits of heating inhaled air have been recognized for many decades and are described in various publications including U.S. Pat. Nos. 3,835,853; 4,325,365; 4,601,287; 4,793,343; and 4,829,997. Inhaled air has been heated with electrical elements as taught in U.S. Pat. Nos. 2,626,343; 3,249,108; 4,601,287; 4,620,537; and 4,793,343 and with heated water as taught in U.S. Pat. No. 5,086,766. Inhaled air has been heated also by devices that obtain heat from portions of the human body rather than external power sources as described is U.S. Pat. Nos. 4,269,183, 4,473,071, 4,671,268, and 4,683,869. The advantage of heating the inhaled air with humid exhaled air has been recognized for many decades. U.S. Pat. Nos. 3,326,214, 3,333,585, 3,814,094, and 4,136,691, 4,478,215 teach heating and humidifying inhaled air with exhaled air by passing the air through an exchanger fabricated from either foil, a nonwoven resilient porous fibrous organic polymeric material, a foraminous heat conductive material, or a continuous strip of wire netting wound helically. In addition, U.S. Pat. Nos. 4,150,671, 4,196,728, 4,200,094, 4,325,365, and 4,458,679 teach heating inhaled air with one of the following: a conduit having heat-exchanging fins; a flexible curved horn of expanding cross section, having a flared internal chamber containing metal mesh; a spool-like member having a series of transverse notches which leave integral fins; an elongated curved housing in which is disposed a plurality of pairs of spaced-apart vanes; or a counter-current medium such as a metal sponge. U.S. Pat. No. 3,835,853 teaches an improved device for heating inhaled air with exhaled air which avoids ice formation at very low temperatures by bypassing a portion of the inhaled air with an automatic valve. Some of these patents, e.g., U.S. Pat. No. 4,478,215, teach that the inhaled air is humidified as well as heated by the exhaled air.
None of the disclosed devices for heating air for breathing is in widespread use. Generally the effective devices are heavy and bulky. In addition, many are relatively costly to produce. Widespread use of a device for warming air that is breathed in requires that the device be relatively small, light weight, and inexpensive. These characteristics demand that the heat exchanger be simple and yet highly efficient and highly effective.